A technology to process an image input from a camera and display the processed image on a display screen is known. For example, there is a camera image processing called AGC (Automatic Gain Control).
In general, AGC refers to the control that makes an image displayed on a display screen not too dark and not too bright, through an electrical circuit or the like, by performing a process of increasing the entire brightness if an average brightness of an image signal input from a camera is weak, while performing a process of decreasing the entire brightness if the average brightness of the image signal is strong. Such a camera image processing technology is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 11-136571 (Patent Document 1).
Specifically, with respect to luminance devoted to all pixels in an image, correction is performed in a manner that if the luminance is low relative to the average luminance, the luminance is heightened by a prescribed % depending on the low luminance level, while if the luminance is high relative to the average luminance, the luminance is lowered by a prescribed % depending on the high luminance level (hereinafter referred to as “average luminance correction”).
As described above, by performing the average luminance correction with respect to the image, too dark part becomes brighter and too bright part becomes darker relative to the average luminance, so that the entire image can give the impression of calm to a user, and as a result, the user can easily view the image.
On the other hand, if the average luminance correction is simply performed in a state where a part of the image is extremely bright relative to other parts, it becomes difficult for the user to see the image.
For example, if light of headlights of a vehicle that travels the opposite lane is contained in a part of a vehicle surrounding image captured by a camera mounted on an outside of the vehicle, the average luminance of the image is calculated to be considerably higher as compared with a case where the light of the headlights is not contained in the image. If the average luminance correction is performed based on the high average luminance, the luminance of the entire image calms down to darken the entire image. Further, the originally dark part is not brightened, and thus it becomes difficult for the user to see the image.
Accordingly, if the luminance devoted to the pixels in the image exceeds a threshold value in the case of calculating the average luminance, the luminance is excluded from the calculation of the average luminance. That is, by performing the average luminance correction through calculation of the average luminance except for the excluded luminance, the problem can be solved that due to the influence of the high luminance in the part of the image, the image after correction becomes dark entirely, and the originally dark part is not brightened. Such a technique is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2004-120203 (Patent Document 2).
However, since a vehicle is put into various situations, the luminance distribution in the image that is successively captured by cameras mounted on a vehicle varies variously, and even if the average luminance is calculated with respect to the image using a constant threshold value, the optimum average luminance correction may not be performed. For example, the luminance of the extremely bright part of the image that is obtained in nighttime corresponds to the proper brightness in the image obtained in the daytime. Because of this, in the case of using the constant threshold value, an appropriate correction may not be performed with respect to the daytime or nighttime image.
Further, recently, a vehicle surrounding image display system has been developed which generates one sheet of vehicle surrounding image that shows the state of the vehicle surroundings through synthesis of images input from a plurality of cameras mounted on a vehicle and displays the generated vehicle surrounding image on a display screen.